Actuators producing mechanical movement of an object in response to the application of electrical power are well known. Among the types of actuators that respond to the application of electrical power to produce mechanical motion are electrothermal actuators. Examples of such actuators are described in U.S. Pat. Nos. 4,759,189, 4,887,429, and 5,203,171, which are incorporated by reference. Within these electrothermal actuators, a closed chamber contains a working fluid. The working fluid is mostly a liquid at ambient temperature and changes phase to become a gas, when heated. That gas phase of the working fluid expands upon continued heating, increasing internal pressure within the chamber. (In the following description, the reference to the working fluid encompasses both of the liquid and gas phases of that fluid, the gas phase expanding upon heating to provide the motive force of the actuator.)
The chamber includes an electrically powered heater that supplies heat to the fluid, in response to an electrical current supplied to the heater. The heat produces the phase change in the working fluid and the pressure increase within the chamber. In response to the increased internal pressure in the chamber, a flexible rolling diaphragm, usually peripherally clamped to the package of the electrothermal actuator, is displaced. The diaphragm displacement pushes a piston that drives a piston rod in a linear direction, increasing the extent of protrusion of the piston rod from the package of the electrothermal actuator. Thus, upon activation and extension of the piston rod, the overall length of the actuator significantly increases.
When electrical power is removed from the heater and pressure in the chamber decreases, the piston rod retracts so that the original overall length of the actuator is restored. Typically, an electrothermal actuator includes a return spring urging the piston to withdraw the piston rod into the package of the actuator. The expansion of the working fluid provides a force that counteracts the restoring force of that return spring.
In many potential applications of electrothermal actuators, there is little space. Thus, the conventional electrothermal actuator cannot be used in these applications.